Analysis of polystyrene production technology

The styrene resin is a series of resins obtained by homopolymerizing or copolymerizing a styrene monomer. In 1998, 77% of the world's styrene was used to produce various types of styrene series resins, compared with 83% in Japan. Commercially available styrene polymers mainly include general-purpose polystyrene (GPPS), impact polystyrene (IPS), expanded polystyrene (EPS resin), acrylonitrile-butadiene-styrene copolymer (aBS), A styrene-acrylonitrile (SaN) copolymer or the like. Several important commercial styrene polymers are basically produced by a bulk, solution, suspension or emulsion process by a free radical chain polymerization mechanism. The diluent bulk process is most commonly used, although some styrenic resins are used. The suspension process (EPS resin) and the emulsion process (aBS resin) are produced, but for economic and other reasons, it is a development trend to use a continuous bulk process whenever possible.

Polystyrene (PS) produced by free radical polymerization is a random polymer with a glass transition temperature of 105 ° C. PS homopolymer is an amorphous brittle material with excellent transparency and processability. A product with a complex shape. IPS is a polymer blend formed by polymerizing styrene in the presence of a polybutadiene rubber or a styrene-butadiene copolymer (rubber particles are dispersed in a PS matrix).

The copolymer of styrene with acrylonitrile, α-methylstyrene and maleic anhydride gives the polymer a high thermal and mechanical properties. The copolymerization of styrene and methacrylate improves transparency and abrasion resistance. The terpolymer of styrene and acrylonitrile and butadiene (aBS resin) has excellent thermal properties, mechanical properties and impact resistance. Styrene copolymer is a bridge between general-purpose resins and engineering resins. It is mainly used in the fields of automotive, electronic appliances and instrument parts, and household appliances. In these applications, nylon (Pa), polycarbonate (PC) and poly are used. Propylene (PP) competes with polyvinyl chloride (PVC).

Industrial production of polystyrene mainly uses two production processes: bulk and suspension. The ontology method is the most important production method. At present, more than 85% of the world's Ps and IPS are produced by continuous bulk process. The continuous bulk production unit generally has one or several production lines with a production capacity of (20-160) kt/a. By improving reactor design, relative molecular mass, and rubber particle size control and devolatilization techniques, the bulk process line can be made larger and more efficient. At present, a large-scale bulk production facility with a single-line capacity of 90 to 138 kt/a has been put into industrial operation, but generally the single reactor capacity is 30 to 50 kt/a.

The suspension method is the second basic production process of polystyrene. The scale of the suspension process is generally smaller than that of the bulk process, and the cleaning time is short during intermittent operation and brand switching. For some products with high heat resistance and high molecular weight grades, they can only be produced by batch suspension polymerization process, but the fixed asset method and continuous production cost of the factory with the same production capacity are lower than the suspension method, so for large Most PS grades are more economical to produce by bulk. At present, the suspension method has generally been replaced by the bulk method and is mainly used for the production of EPS.

Styrene can be used as both an electron donor and an electron acceptor. The polymerization process can have four different mechanisms: radical polymerization, negative ion polymerization, positive ion polymerization, and coordination polymerization. Several mechanisms of styrene polymerization have their own characteristics: the radical initiation, growth and termination processes occur simultaneously, so a broad molecular weight distribution (Mw/Mn>2) can be obtained, and various termination pathways make their end groups. It is versatile, and the molecular weight of the polymer is not very demanding on the reactant feed; the chain initiation, growth and termination of the negative ion polymerization mechanism occur one after another, and the molecular weight distribution of the polymer is narrow (Mw/Mn<1.1=, control chain termination The step can control the end group structure of the polymer chain, but it is required that the feed in the polymerization reaction must be purified; since the styryl carbon cation is unstable, the molecular chain is quickly terminated, so the positive ion polymerization mechanism is difficult to produce high molecular mass. The polymer, and the feed of the positive ion polymerization must also be purified; the metal compound used in the Ziegler-Natta coordination polymerization enables the polymerization to proceed in a stereoregular manner. Therefore, it is possible to produce a high melting point and high crystallinity. Stand up the PS, but the feed must be purified in the polymerization. The free radical polymerization is mainly used in industrial production. The first reason is the requirement of the feed monomer. Not high, the second is that the initiator has little effect on the properties of the polymer, so that it is not necessary to remove the residual initiator from the polymer.

The free radical polymerization of the PS resin can be carried out only by heating (above 100 ° C), and the polymerization reaction is generally divided into three stages of chain initiation, chain growth and chain termination, and the polymerization conversion ratio is generally 70% to 85%. As the conversion of styrene increases, the polymerization temperature can be raised from 100 ° C to 220 ° C. In addition, the polymerization can also be initiated by an initiator, and the polymerization can be carried out more rapidly in the presence of an initiator such as benzoyl peroxide. Thermal initiation and chemical initiation are used industrially to produce GPPS and HIPS. Since the styrene polymerization reaction is a large amount of exothermic reaction (67.4 KJ/mol), good mixing and effective heat removal measures are extremely high for the polymerization reaction at a reaction rate of 10% to 20% polymer/h. important. For the polymerization carried out at a reaction rate of 20% polymer / h, good mixing and effective heat removal measures are extremely important.

The number of radicals and the reaction temperature have an influence on the chain transfer and the rate of chain termination. Therefore, control of the chain length (relative molecular mass) can be achieved by controlling the reaction conditions such as the amount of the initiator injected, the injection rate, and the polymerization temperature. Increasing the concentration of the initiator while the reaction temperature is kept constant, a large number of polymer chains are initiated and terminated, resulting in a decrease in the average relative molecular mass. Conversely, if the reaction temperature is raised and the radical supply is kept constant, the polymer chain continues to increase, and the polymerization rate and the average relative molecular mass increase. Generally, the relative molecular mass of the injection molding grade and the extrusion grade is about 160,000 to 180,000, the relative molecular mass distribution coefficient is 2-4, and the relative molecular mass is up to 400,000. The initiator system must be selected to suit the polymerization rate, residence time, relative molecular mass distribution, and residual monomer content within a defined reaction temperature range.

Impact polystyrene is produced by graft copolymerization of styrene and rubber. The obtained product consists of a dispersed rubber phase and a continuous PS phase, and the introduction of rubber improves the toughness and impact resistance of the PS. In order for HIPS to have a high impact strength over a wide temperature range, the rubber used must have a glass transition temperature below -50 °C. Polybutadiene rubber (glass transition temperature -80 ° C) is the most commonly used impact modifier for styrene plastics. Allyl hydrogen atoms and weakly reactive double bonds provide the desired degree of grafting and crosslinking. There have also been reports of the use of other rubbers such as acrylate rubber, ethylene-propylene-diene rubber, polyisoprene rubber, etc., but industrialization has not been fully realized due to factors such as low chemical activity of these rubbers and unsuitable glass transition temperatures.

The SaN resin is formed by block copolymerization of styrene and acrylonitrile, and the polymerization process may be an emulsion method, a suspension method or a bulk method. The content of acrylonitrile in the copolymer is about 15%. The preparation process of aBS resin is to prepare polybutadiene latex of different particle size by floating liquid method, and then carry out styrene-acrylic block copolymerization in emulsion, and simultaneously The copolymerized polybutadiene rubber particles, and then the terpolymer is blended with the SaN polymer. Since the blend SaN can be prepared by the emulsion method, the suspension method and the bulk method, respectively, the SaN and styrene are copolymerized. The preparation process of aBS resin blended by materials is called emulsion grafting emulsion SaN blending process, emulsion grafting suspension SaN blending process, emulsion grafting bulk SaN blending process.